Authorities were investigating a "female in distress in the canyon" call when Jack was found.
Her friend, 19-year-old Nicholas Cendoya, was found alive just before sundown Wednesday. He was located in thick brush by a hiker who was unrelated to the search. He was rushed to Mission Hospital in Mission Viejo, where he is recovering in intensive care.
"He's a healthy young man, and he's pulling through. We're treating as we can," said Dr. Matthew Kaplan with Mission Hospital.
Cendoya was not able to tell authorities when he last saw his friend. When rescuers found Cendoya, they said he appeared to be dehydrated, disoriented and a bit confused.
"He has asked about her, and wants to know if we found her, how she's doing. He's obviously concerned," said Lt. Jason Park of the Orange County Sheriff's Department.
The search for the two young hikers began Sunday when they called 911 for help around 8:30 p.m. and told dispatchers they were hiking on a trail but couldn't find their way back down the rugged terrain. They also said they ran out of water. Their cellphone batteries died before police could trace a signal.
The search on Thursday focused on the area where Cendoya was found, which is about a half-mile south from the Trabuco Canyon trail head.
One problem crews faced was thick brush. Officials said even when the rescue helicopter was over Cendoya, rescuers briefly lost sight of him because the brush was so thick.
Meantime, Cendoya's family said they're very grateful that he has been found alive.
"We just want to thank God, and we're very happy. Thank you very much to everybody that volunteered and helped out," said Patricia Sanchez, Cendoya's cousin.